1. Field of the Invention
The present invention relates to an amplifying circuit.
The invention is particularly concerned with a circuit usable for a measuring instrument such as oscilloscopes.
In spite of large amplitude inputs applied or resistance irregularities of resistors employed therein, the amplifying circuit can deliver distortionless outputs.
Regardless of fluctuations of the environmental temperature or changes of voltages or currents of power supplies, the amplifying circuit can output distortionless waveforms.
2. Description of the Prior Art
In an oscilloscope, waveforms to be observed are large frequency bandwidth signals from DC to high frequency. In the current oscilloscope, it is required to observe signals of several hundred MHz to several GHz or high repetition rate pulses.
An amplifying circuit employed in such an oscilloscope is usually included in an integrated circuit. The integrated circuit satisfies requirements to observe of large bandwidth and high repetition rate signals.
The amplifying circuit being in the integrated circuit includes, generally, differential amplifiers. In order to amplify large bandwidth and high repetition rate signals, npn transistors having excellent high frequency characteristics are employed in the differential amplifier. Many stages of the differential amplifiers including npn transistors are connected in series so as to constitute the amplifying unit. In the amplifying unit, there is a problem that the output potential is shifted up to positive in potential by the connection in series.
Shown in FIG. 1 is a circuit diagram of a prior art amplifying unit. The unit is constituted of two stages of differential amplifiers cascadedly connected. In FIG. 1, elements 101 and 102 are npn transistors to form a common emitter differential amplifier 100. Elements 201 and 202 are npn transistors to form a common emitter differential amplifier 200L.
Elements 103 and 104 are negative feedback resistors in the differential amplifier 100. Each of feedback resistors 103 and 104 is connected in series between emitters of the transistors 101 and 102.
Elements 203 and 204 are negative feedback resistors in the differential amplifier 200L. Each of feedback resistors 203 and 204 is connected in series between emitters of the transistors 201 and 202.
Elements 105 and 106 are load resistors in the differential amplifier 100. Elements 205 and 206 are load resistors in the differential amplifier 200L. Elements 107 and 207 are constant current sources of differential amplifiers 100 and 200L.
The element 151 is a pair of differential input terminals of an amplifying unit 150L including two stages of differential amplifiers 100 and 200L. The element 152 is a pair of differential output terminals of the amplifying unit 150L.
In the differential amplifier 100, each of load resistors 105 and 106 is connected between a constant positive voltage source Vcc and each of collectors of transistors 101 and 102.
In the differential amplifier 200L, each of load resistors 205 and 206 is connected between a constant positive voltage source Vcc and each of collectors of transistors 201 and 202.
The constant current source 107 is connected between a center junction of two feedback resistors 103 and 104 connected in series and a constant negative voltage source VEE. The constant current source 207 is connected between a center junction of two feedback resistors 203 and 204 connected in series and a constant negative voltage source VEE.
The collector of the transistor 101 in the differential amplifier 100 is connected to a base of the transistor 202 in the differential amplifier 200L. The collector of the transistor 102 is connected to a base of the transistor 201 in the differential amplifier 200L.
In the amplifying unit 150L of FIG. 1, the relation between the input voltage Vicom and the output voltage Vocom can be shown as follows.
Vocom greater than Vicom
Therein, Vicom is a common-mode voltage applied to the input terminals 151 and Vocom is a common-mode voltage delivered from the output terminals 152.
In each of the differential input of the input terminals 151 and the differential output of the output terminals 152, a voltage change in common-mode is so called the common-mode voltage.
As already stated, the amplifying circuits in measuring instruments such as oscilloscopes amplify signals from DC to high frequency. In such a large bandwidth, it is desirable that the common-mode output voltage is 0V. It is, therefore, required that the common-mode output voltage Vocom is kept 0V or so.
In FIG. 2, there is shown a circuit diagram of another prior art amplifying unit. A differential amplifier 200M of the second stage in the amplifying unit 150M includes two pnp transistors 208 and 209. It is able to keep the same output voltage Vocom of the terminals 152 as the input voltage Vicom of the input terminals 151 in common-mode.
However, pnp transistors are, generally, inferior to npn transistors in high frequency characteristics. The amplifying unit 150M including pnp transistors 208 and 209 can not, therefore, obtain the same large bandwidth as that of npn transistors.
In the amplifying circuit used for measuring instruments like an oscilloscopes, a level-shift circuit is employed. The circuit shifts the DC level of the common-mode output voltage Vocom to 0V approximately. The circuit is connected to the output stage of an amplifying unit.
In FIG. 3, there is shown a circuit diagram of a prior art amplifying circuit with a level-shift circuit. The level-shift circuit 300 is appended to the amplifying unit 150L of FIG. 1. In FIG. 3, elements similar to those previously described with reference to FIG. 1 are denoted by the same reference numerals.
A pair of differential output terminals 162 is output terminals of the amplifying circuit 160L with the level-shift circuit. The level-shift circuit 300 is constituted of a couple of two level-shifters having the same composition. A level-shifter includes an npn transistor 301, a diode group 303 of n diodes connected in series and a constant current source 305. Another level-shifter includes an npn transistor 302, a diode group 304 of n diodes connected in series and a constant current source 306. Each of diode groups 303 and 304 containing one diode or more connected in series functions as a constant voltage diode.
A base of the transistor 301 is connected to the collector of the transistor 202 in the differential amplifier 200L. A base of the transistor 302 is connected to the collector of the transistor 201 in the differential amplifier 200L. Each of bases of transistors 301 and 302 is connected to the output terminals 152 of the differential amplifier 150L.
A collector of the transistor 301 is connected to a positive voltage source VCC. A collector of the transistor 302 is connected to a positive voltage source VCC.
An emitter of the transistor 301 is connected to an anode of the first diode in the diode group 303. An emitter of the transistor 302 is connected to an anode of the first diode in the diode group 304.
A cathode of the last diode in the diode group 303 is connected to one end of the constant current source 305. A cathode of the last diode in the diode group 304 is connected to one end of the constant current source 306. Another end of each of constant current sources 305 and 306 is connected to each of negative voltage sources VEEs.
The output of the amplifying circuit 160L with a level-shift circuit 300 is delivered from each of cathodes of the last diodes in diode groups 303 and 304. The cathodes of the last diodes are connected to a pair of differential output terminals 162.
The base-emitter voltage of each of transistors 301 and 302 is Vbe. The anode-cathode voltage (forward-voltage) per one diode in diode groups 303 and 304 is Vf. A voltage shifted by the level-shift circuit 300 is representable as Vsft by the following equation.
Vsft=Vbe+nxc3x97Vf
The desired number of n diodes is, therefore, settled in each of diode groups 303 and 304 so as to obtain the desired voltage Vsft to be shifted.
However, the amplifying circuit 160L with a level-shift circuit shown in FIG. 3 has following defects. Namely, the base-emitter voltage Vbe of each of transistors 301 and 302 changes by environmental temperature changes. Similarly, the forward-voltage Vf of each of diodes in diode groups 303 and 304 changes, too. The shifted voltage Vsft cannot be, therefore, kept constant.
For example, each of base-emitter voltage Vbe of transistors and forward-voltage Vf of diodes is 0.7V(Vbe=Vf=0.7V). The number of n diodes in each of diode groups 303 and 304 is 5(n=5). In such a case, the shifted voltage Vsft is 4.2V from the above-mentioned equation.
Each of the base-emitter voltages Vbes and the forward-voltages Vfs changes according to temperature changes. Suppose the voltage Vbe and the voltage Vf change at xe2x88x922 mV/xc2x0 C. being typical value, the shifted voltage Vsft by a temperature change of 100xc2x0 C. is 5.4V. The shifted voltage Vsft changes by 1.2V.
Moreover, the common-mode output voltage Vocom from the output terminals 162 may be changed by the voltage change of the positive voltage source Vcc or the current change of the constant current source 207.
It is desired that the common-mode output voltage Vocom of the output terminals 162 is fixed to 0V approximately. For the purpose, it is required to prevent from influences by temperature changes and voltage changes or current changes of power supplies.
In FIG. 4, there is shown a circuit diagram of another prior art amplifying circuit 160M with the level-shift circuit 300M. In FIG. 4, elements similar to those previously described with reference to FIG. 3 are denoted by the same reference numerals. The followings are different points from the constitution of the amplifying circuit with the level-shift circuit 300 shown in FIG. 3.
In the level-shift circuit 300M, two resistors 307 and 308 are connected in series between cathodes of the last diodes in each of diode groups 303 and 304. Resistors 307 and 308 have the equal resistance. The resistance is 1 or several kxcexa9 and is enough higher than that of a load resistor to be connected between output terminals 162. Each of resistors 307 and 308, of which resistance is enough higher than that of the load resistor, effects as no load resistor.
A center junction 309 of the resistors 307 and 308 is connected to a negative input terminal of an operational amplifier 30. A positive input terminal of the operational amplifier 30 is grounded. An output of the operational amplifier 30 is connected to a common junction 210 of the load resistors 205 and 206 in the differential amplifier 200 of the second stage.
According to the circuit 160M, the average voltage of the output from the output terminals 162, i.e., the common-mode output voltage Vocom appears at the common junction 309 of two resistors 307 and 308. The common-mode output voltage Vocom is varied by the base-emitter voltage Vbe of each of transistors 301 and 302 influenced with temperature changes. The common-mode output voltage Vocom can be changed by the forward-voltage Vf of each of diodes in diode groups 303 and 304, as the forward-voltage Vf is effected by temperature changes, too.
The common-mode output voltage Vocom is applied to the negative input terminal of the operational amplifier 30. The output of the operational amplifier 30 is negatively feedback to the common junction 210 of the load resistors 205 and 206. The common-mode output voltage Vocom can be, therefore, kept at the ground voltage of 0V. The ground voltage is caused by the grounded positive input terminal voltage of the operational amplifier 30.
Therefore, according to the amplifying circuit 160M with a level-shift circuit shown in FIG. 4, the common-mode output voltage Vocom from the output terminals 162 can be stabilized. Even if the shifted voltage Vsft of the level-shift circuit 300M is influenced with temperature changes, the common-mode output voltage Vocom can be stabilized.
However, there is a problem to be solved in the amplifying circuit 160M with the level-shift circuit. When being applied with a large amplified input signal, the differential amplifier 200 is saturated. In the output from the output terminals 162, the output signal waveform is, thereby, distorted.
In FIG. 5, there are shown operations of the amplifying circuit 160M with the level-shift circuit. When a large amplified pulse is applied at the input terminals 151, the differential amplifier 100 and 200 are saturated.
Each of waveforms 401 and 402 shown in FIG. 5(A) is obtained from each of the output terminals 162 when the feedback control of the operational amplifier 30 is not employed. Each of waveforms 403 and 404 shown in FIG. 5(B) is obtained from each of the output terminals 162 when the operational amplifier 30 is employed as shown in FIG. 4.
In FIG. 5(A), the common-mode output voltage Vocom is shifted. The common-mode output voltage Vocom is nearly 1.5V. However, the waveform 401 in positive direction and the waveform 402 in negative direction are equal in amplitude. The unbalance between the waveforms is at a minimum.
On the other side, in FIG. 5(B), the common-mode output voltage Vocom is not almost shifted. Namely, the common-mode output voltage Vocom is nearly 0V. However, the waveform 403 in positive direction and the waveform 404 in negative direction are not equal in amplitude. The waveforms 403 and 404 are unbalanced. Because, the distortions have been caused on each of waveforms 403 and 404 obtained at the output terminals 162.
The waveform distortions in the circuit of FIG. 4 are based on the following reasons. When differential amplifiers 100 and 200 are saturated, the unbalance is caused in the amplitudes of positive and negative directions at the output terminals 162. Therefore, the voltage at the center junction 309 of two resistors 307 and 308 is not equal to the common-mode output voltage Vocom. The signal applied to the negative input terminal of the operational amplifier 30 is inconstant as shown by the dotted line 405 of FIG. 5(B). Consequently, in the range of the response time of the operational amplifier 30, the voltage of the common junction 210 of two load resistors 205 and 206 is varied and it causes the waveform distortions.
The larger the open-loop gain of the operational amplifier 30 is, the more remarkable the phenomena become. For example, in an oscilloscope, the observation of waveforms may be impossible during several micro seconds after a large amplitude input signal being applied to the input terminals 151 of the amplifying circuit 160M.
In an oscilloscope, for observing rising and falling portions of a pulse, the pulse amplitude may be amplified enough. Moreover, for observing a waveform near 0V in detail, the amplitude of the pulse may be magnified enough and displayed. In such a case, differential amplifiers 100 and 200 are saturated.
In an oscilloscope employing the amplifier such as the amplifying circuit 160M with a level-shift circuit, it is required to take countermeasure against waveform distortions as much as possible so as to observe distortionless waveforms. In spite of excessive input by which differential amplifiers 100 and 200 are saturated, it is important to be able to display distortionless waveforms. Especially, in the current wide band oscilloscope, waveforms may be sometimes observed in a range of a few nano seconds. In the case, there is a serious problem that it is impossible to observe waveform during several micro seconds after an excessive input applied to differential amplifiers 100 and 200. It is important to enable the oscilloscope to display waveforms as quickly as possible after the excessive input.
Besides, waveform distortions of the output from the amplifying circuit 160M with the level-shift circuit may be caused by the irregularity of resistances of load resistors 205 and 206. Waveform distortions are also caused by the irregularity of resistors 307 and 308. In spite of differential amplifiers 100 and 200 being not saturated, waveform distortions of the output are effected by those irregular resistances.
The output from the amplifying circuit 160M shown in FIG. 4 is not theoretically influenced with temperature changes or voltage or current changes of power supplies. The common-mode output voltage Vocom from the output terminals 162 can be kept constant. However, differential amplifiers 100 and 200 may be, actually, saturated by the excessive input. The irregularity of resistances may happen in load resistors 205 and 206 or resistors 307 and 308. In such a case, the amplifying circuit 160M shown in FIG. 4 has the defects that waveform distortions of the output are effected.
An object of the invention is to provide a new amplifying circuit with a level-shift circuit without waveform distortions even if differential amplifiers employed therein are saturated.
Another object of the invention is to provide a novel and highly precise amplifying circuit with a level-shift circuit without waveform distortions at the output even if resistors used in the differential amplifiers have irregular resistances.
A further object of the invention is to provide a new and highly precise amplifying circuit with a level-shift circuit without waveform distortions at the output in spite of temperature changes or voltage and current changes of power supplies.
A further object of the invention is to provide a novel amplifying circuit with a level-shift circuit which is suitable for constituting monolithic integrated circuits.
In the amplifying circuit with a level-shift circuit according to the invention, the amplifying circuit includes an amplifying unit, a level-shift circuit, a DC-dummy, a level-shifter and an operational amplifier.
An amplifying unit includes at least one differential amplifier cascadedly connected. A differential amplifier of the first stage has a pair of input terminals to which a pair of differential input signals is applied. A differential amplifier of the last stage has a pair of differential output terminals to obtain the differential output.
A level-shift circuit is consisted of a couple of level-shifters. The level-shift circuit is connected to the pair of differential output terminals of the differential amplifier of the last stage in the amplifying unit. The level-shift circuit shifts the DC level of the differential output from the amplifying unit and delivers the shifted differential output.
A DC-dummy operates as a dummy of the DC operation of the differential amplifier of the last stage in the amplifying unit. Therefore, the DC-dummy has the power supply in common with the differential amplifier of the last stage.
The level-shifter shifts a level of the output of the DC-dummy to obtain the dummy output level-shifted. The constitution of the level-shifter is similar to one of a couple of level-shifters in the level-shift circuit.
The negative input terminal of the operational amplifier is maintained at grounded voltage or a fixed voltage. Therefore, the operational amplifier controls so as to supply the power to the differential amplifier of the last stage in the amplifying unit and the DC-dummy in common.
According to the above composition, the shifted voltage of the level-shifter equals to that of the level-shift circuit. Then the operational amplifier controls so as to supply the power. The output voltage of the level-shifter is, therefore, kept constant. Consequently, the common-mode output voltage Vocom delivered from the level-shift circuit is kept constant.
Furthermore, even if the differential amplifiers in the amplifying unit is saturated by an excessive input, only the DC voltage appears in the DC-dummy and the level-shifter. A stable feedback operation is obtainable so as to keep the Vocom constant. Therefore, no waveform distortion is caused in the output of the amplifying circuit with the level-shift circuit.